For some time, CRT manufacturers have used filters to obtain contrast enhancement for full color displays. Useful filters transmit light of frequencies corresponding to the phosphor emission frequencies and absorb all other unwanted light. This substantially reduces glare in full sun applications when combined with a front surface AR coating. The most commonly used phosphors for color displays at present P43 and P22. Their primary emission bands are 544 nm (P43), and 445 and 620 nm (P22). Thus, the optimum contrast enhancement filter would have transmission windows of any desired level around these three wavelengths and would be opaque at all other wavelengths.
The first types of contrast enhancement filters utilized were composite filters consisting of two or more glass types such as, for example, a Schott neutral density filter type S-4020 together with a multiband Didymium filter, Schott S-8801. Composite filters often have transmission characteristics unobtainable in a single glass because of colorant incompatability.
The major disadvantage of a composite filter is the high cost of fabrication. This arises from the necessity to prepare a high optical finish on each of the filter surfaces and because the laminate must be bonded together with optical cements to eliminate Fresnel losses. Another significant disadvantage is the increased weight of a composite filter compared to a single glass filter. A typical prior art composite filter is disclosed in U.S. Pat. No. 4,245,242.
Because of the high fabrication cost of composites, single glasses which combine all required transmission characteristics (as much as possible) have been developed. Relevant prior-art glasses are disclosed in U.S. Pat. Nos. 4,521,524; 3,143,683; 4,520,115 (concerned with X-ray absorption); 4,405,881; and 4,390,637. The colorant combinations used to achieve contrast enhancement in the prior art generally involve:
(a) Nd.sub.2 O.sub.3 +Pr.sub.2 O.sub.3 in combination to give multiple narrow bandwidth transmission windows; PA0 (b) Fe.sub.2 O.sub.3 and CuO to absorb red frequencies outside the phosphor range; PA0 (c) NiO and CoO to adjust overall transmittance in the visible range; and PA0 (d) other optional ingredients to fine-tune desired properties.
For example, components (d) have been added to absorb UV and near-UV light, e.g., Se (U.S. Pat. No. 3,143,683) or CeO.sub.2 (U.S. Pat. No. 4,521,524).
These colorants are often incompatible and do not give the maximum contrast efficiency. Specifically, the combination of iron (required as Fe.sup.2+) and copper (required as Cu.sup.2+) is incompatible with respect to oxidation state. The former requires very reducing atmospheric conditions during manufacture, while the latter requires very oxidizing conditions. An additional serious problem is the use of both neodymium and praseodymium. Neodymium alone presents no problems. However, praseodymium has a strong absorption band at the major emission line (445 nm) of the P22 phosphor commonly used in color CRT displays. Its use is therefore contraindicated since its incorporation tends to degrade the contrast enhancement efficiency of filter used in such applications.
An additional complication of the colorants used in the prior art glasses, particularly for U.S. Pat. No. 4,521,524, is that the base composition most useful for stabilizing the colorants tends to have a high refractive index, typically greater than 1.57. This is undesirable. The front surface AR coatings used on these filters are most efficient if the refractive index of the base glass is less than 1.57.
The most desirable contrast enhancement filter, therefore, would be a single glass filter utilizing compatible colorant combinations which do not absorb undesirably at the phosphor wavelengths of interest and which has a refractive index Nd less than 1.57.